The effective photo-electric absorption cross section index or factor (PEF or Pe) can provide useful information about the lithology of a rock formation. It is important to have measurements of PEF for actual rock samples for which detailed elemental analysis is known in order: (1) to determine whether the experimental values of PEF agree with theory; (2) to test the additivity of absorption cross sections for mixtures; (3) to determine the accuracy of the common approximate formula for PEF, which depends only on the atomic number Z of the atoms in the sample; and (4) to study the effect of scattering corrections on the determination of PEF.
Knowledge of PEF is an important factor in effectively using the widely used Litho-Density Tools, as an example, the one by the commercial service companies manufactured by Schlumberger. A discussion of the Litho-Density tools is included with Watson, "Numerical Stimulation of the Litho-Density Tool Lithology Response", SPE 12051, (1985) which is incorporated by reference herein for all purposes.
Heretofore, it has been necessary to perform detailed elemental analysis on rock samples in order to determine PEF. Based on a detailed elemental analysis it is readily possible to compute PEF. However, such analyses are time consuming and costly.
It is desirable, therefore, to devise a method and apparatus for determining PEF of rock samples which is quick and inexpensive.